Sunday, March 12, 2006

Sunday Porsche Blogging: the New Porsche Turbocharger

As Porsche prepares to release its new type 997 Turbo, they are starting to show the car off to the press. The Porsche factory magazine Christophorus has an interesting piece on the turbocharger itself(1). The new turbocharged version of the 911 Carrera utilizes one of the fifteen pound devices on each of the two exhaust streams, one on each side of the flat six cylinder engine. For this new car Porsche has developed the first turbocharger to be used in a gasoline engine that uses variable-turbine geometry, meaning that the turbocharger is always working and thus ready to immediately increase power by working harder. This is a big deal.

The turbocharger for piston engines was patented by Swiss engineer Alfred Buchi in 1905, a time when the full potential of the device was not realized due to the materials available back then. Turbochargers increase power by increasing the amount of the air/fuel mixture consumed by the engine; they greatly increase the amount of air forced into the intake manifold. This attribute of higher manifold pressure was the attraction for one of its first major applications; piston airplane engines used in the low pressure environment of high altitude before and during WWII.

Turbochargers work by placing a powered impellor rotating at very high speeds that acts as a compressor in the path of the air going into the engine, thus increasing the airflow’s speed and pressure. If the impeller was powered by a belt off of the engine it would be a supercharger, or Kompressor in Mercedes-speak. Instead the turbocharger saves engine power by using a shaft attached to an impellor placed in the fast moving, highly heated stream of exhaust gasses. This proximity to the exhaust has caused serious heat issues with turbochargers over the years; that combined with the fact that compressed air heats up are the reasons that intercoolers have been so popular with turbocharged applications. It is desirable both in terms of power output and engine longevity to have a cooler fuel/air input charge.

Diesel engines have used turbochargers for years; this makes sense as diesels are powered by pressure driven explosions in the cylinders. Exhaust gas temperatures of Diesel engines run cooler than Otto (gasoline) engines, 1350 degrees F vs. 1850 degrees F, making the construction of turbocharging devices easier in the diesel world. Diesels have been using turbochargers with variable turbine technology for some time now due to the lower temperature environment; the metallurgy was not all that expensive as there already were jet engine and space travel applications. The differing expansion rate of different metals was the difficult problem that Porsche engineers had to surmount in order to build this new turbocharger.

When I first heard about this variable-turbine geometry I assumed that variable part referred to the actual vanes on the exhaust impeller, with the vanes changing their angles of attack depending on how forceful the passing gasses are. In fact it refers to a device that directs the exhaust stream through the impellers. The device, consisting of eleven small blades, expands like tiny lobster tails into the exhaust stream to capture more of the exhaust stream at times when the exhaust stream is less powerful, i.e. lower RPMs. By contrast the device retracts to capture less of the stream at higher engine speeds due the lower relative power needs to keep the turbocharger impeller spinning at optimum levels. This is the portion of the turbocharger that needed the metallurgy breakthroughs.

This is a big deal because it allows the turbocharger to spin at optimum levels at a wider range of engine speeds, especially low speeds. Traditional turbochargers need higher engine speeds with the corresponding higher exhaust gas speeds in order to spin fast enough to push the amount of air needed to boost the intake pressure. This often produced dramatic “turbo lag”, the time that the turbocharger took to spool up fast enough to make a difference to the engine’s power output. In some cars the power would come on with such a surge that the driver needed some skill to control the vehicle. Because the exhaust can account for a loss of up to thirty-percent of an engine’s power, the turbocharger allows the engine to capture some of this lost power and put it to use, allowing for a smaller, lighter and more efficient engine relative to power output. The turbo lag problem has been a definite marketing issue, a smaller engine that doesn’t get good power until it reaches 3000 RPM and then has a rush of power is much less comfortable to use than a larger engine that puts out good usable power at lower engine speeds but gets worse gas mileage. In addition to being more efficient fuel wise, the smaller engine also weighs less, another efficiency that leads to better gas mileage. The smaller size also offers a host of other advantages including engine placement.

This new Porsche turbocharger is a big deal because it dramatically advances turbocharger technology and if this new technology can be produced in a cost efficient manner it seems likely that we will see more, and more usable turbocharged automobiles. This technology could lower the amount of energy that we use for each mile traveled. The new Porsche turbocharger also has a usable range large enough to find use in the gas engine portion of hybrid vehicles, making those vehicles even more efficient.

(1)Christophorus, The Porsche Magazine; number 318, February/March 2006, pages 16-27


Post a Comment

<< Home